The principle of seismic surveying is that a source of seismic energy is caused to produce seismic energy that propagates downwardly through the earth. The downwardly-propagating seismic energy is reflected by one or more geological structures within the earth that act as partial reflectors of seismic energy. The reflected seismic energy is detected by one or more sensors (generally referred to as “receivers”). It is possible to obtain information about the geological structure of the earth from seismic energy that undergoes reflection within the earth and is subsequently acquired at the receivers.
In practice, a seismic surveying arrangement comprises an array of sources of seismic energy. This is because it is necessary to generate sufficient energy to illuminate structures deep within the earth, and a single seismic source generally cannot do this.
Sources of seismic energy are known which emit seismic energy at more than one frequency. Examples of such seismic sources are vibrator sources, which emit seismic energy in a frequency range of, for example, from 5 or 10 Hz to 100 Hz. When such a vibrator source is actuated, seismic energy is emitted over a finite time period, and the frequency of the emitted energy changes during the period over which seismic energy is emitted. For example, the frequency of the emitted energy may increase monotonically during the period over which seismic energy is emitted. The process of operating a vibrator source of seismic energy to cause emission of seismic energy over the frequency range of the vibrator will be referred to herein as “sweeping” the vibrator, and the step of initiating a vibrator sweep will be referred to as “actuating” the vibrator. Each emission of seismic energy from a vibrator is known as a “shot”. The time period over which seismic energy is emitted by the vibrator source will be referred to as the “sweep time”, and the “sweep rate” is the rate at which the frequency changes over the sweep time (a linear sweep rate is generally used in practice).
A seismic vibrator source for use on land consists generally of a baseplate in contact with the ground. Seismic energy is transmitted into the ground by applying a vibratory force to the plate, and this is done by applying a control waveform known as a “pilot sweep” to the vibrator control mechanism. The pilot sweep is generally a constant amplitude swept frequency signal, although it tapers off at each end to allow the amplitude of the vibration to be ramped up and down at the start and finish of the sweep respectively. In practice the waveform applied to the ground by the plate is not exactly the same as the pilot waveform; in particular, as well as applying a force at the desired frequency at any particular time (known as the “fundamental frequency”), the vibrator also applies a force at integer multiples of the fundamental frequency (known as “harmonics”).
Marine vibrator sources of seismic energy are also known. The are again swept so as to emit seismic energy over a range of frequencies.
When a seismic vibrator source is actuated to emit seismic energy, the seismic energy incident on a receiver is recorded for a pre-determined period from the start of the sweep time of the source. The time from the end of the sweep time to the end of the recording period is generally known as the “listening time”, and data is acquired at a receiver from the start of the sweep time to the end of the listening time. The data acquired at a receiver in consequence of actuation of a source is then processed, for example by cross-correlating the acquired data with the pilot sweep of the source to produce a record that is the length of the listening time.
FIG. 1 is a schematic illustration of the process of a conventional seismic survey that uses an array of land vibrator sources. At time T0, one seismic source in the source array is actuated to start its sweep. In this example, the vibrator sweep time has a duration S, and the frequency of seismic energy emitted by the vibrator increases monotonically from a frequency f0 at time T0 to a frequency f1 (f1>f0) at the conclusion of the sweep (at time T0+S). The sweep time is followed by the listening time, so that the overall time of the process of actuating and sweeping the source and listening at a receiver for seismic energy is S+L, where L is the duration of the listening time.
In a conventional seismic survey the sources are actuated such that a receiver will receive seismic energy from only one source in any given listening period. The minimum delay between the start of two vibrator sweeps in such a survey is therefore the sum of the sweep time S and the listening time L. The listening time L is made sufficiently large that all seismic energy required at a receiver in a particular listening period was emitted during the sweep time immediately preceding that listening period.
The conventional seismic surveying process has the disadvantage that it can be slow, owing to the need for the minimum time delay between the starts of two vibrator sweeps to be the sum of the sweep time and the listening time. One known attempt to reduce the time required to carry out a seismic survey is the “slip-sweep” acquisition technique. In the slip-sweep technique the minimum time delay between the starts of two subsequent vibrator sweeps is only the listening time, not the sum of the sweep time and the listening time. The record length after cross-correlation is the length of the listening time.
The slip-sweep technique is illustrated in FIG. 2. As in the method of FIG. 1, one seismic source in the source array is actuated to start its sweep at time T0, the vibrator sweep time has a duration S, and the sweep period is followed by a listening time L. The time T1 at which a second source is actuated to start its sweep is not however required to satisfy T1>T0+S+L, but is only required to satisfy T1>T0+L. Since the minimum time delay between actuation of two sources in the slip-sweep technique is only the listening time, not the sum of the sweep time and the listening time, the slip-sweep technique allows the time to complete a seismic survey to be reduced. It has the disadvantage, however, that harmonics of the fundamental frequency generated by one vibrator are present on the seismogram recorded by one or more preceding vibrators.
A further known surveying technique is the technique of “simultaneous shooting”. In the simultaneous shooting method two or more seismic sources disposed at respective shot locations are actuated to start their sweeps at the same time. The seismic energy acquired at a receiver will therefore contain events arising from seismic energy emitted by all sources. In order to allow the events corresponding to each source to be separated out from one another, each vibrator must sweep, at its shot location, at least as many times as there are vibrators in the group, and the recorded data are then manipulated algebraically to separate the events corresponding to each source. Typically each vibrator will sweep for the same length of time, at the same sweep rate, and over the same frequency range, but the phase relationship between vibrators changes from one record to another. In the case of a group of two vibrators, for example, one suitable scheme would be for the two vibrators to sweep in phase during the first record and to sweep 180° out of phase during the second record. The mean of the two signals acquired by a receiver gives the signal arising at that receiver from actuation of one vibrator, and half the difference of the two signals acquired by a receiver gives the signal arising at that receiver from actuation of the other vibrator.